1. Field of the Invention
The present invention relates in general to methods of static liquid mixing and more particularly to static mixing of liquid systems comprising a carrier fluid and one or more diluents. Such methods of mixing are most suitable for applications in semiconductor industry for dilution or concentration of etching, cleaning, or polishing solutions in semiconductor wafer fabrication.
2. Prior Art
Fluid mixing is employed in numerous applications with the goal to achieve uniformity of various physical and chemical properties such as density, temperature, viscosity, concentration, etc.
Fluid mixing could be accomplished by various methods. These methods may be broken down into three major categories: 1) mechanical agitation; 2) gas bubbling; and 3) static mixing.
Mechanical agitation involves usage of moving parts and therefore the reliability of devices that utilize it is inferior to that of devices utilizing static mixing methods.
Bubbling gases through liquids does not provide uniformity of mixed fluid parameters that could be achieved by other mixing methods.
The present invention relates to static mixing methods and devices. State of the art in static mixing is taught in Chemical Engineering courses, see for example chapter on static mixers in CHEMICAL ENGINEERING by J. M. Coulson and J. F. Richardson with J. R. Backhurst and J. H. Harker, Sixth Edition, Butterworth-Heinemann Publishing House, December 1999, volume 1, pp. 307-310. There, numerous static mixers are described comprising stationary helical blades contained within a pipe. Various combinations of lattices placed within a pipe are also described. Helical blades and lattices serve for cutting and twisting the flow to achieve better mixing. Multiple divisions and recombinations of fluid flow within a static mixer containing the above-mentioned elements (blades and/or lattices) secure homogenous mixing. Identified in that teaching are the most important characteristics of static mixing, namely a) mixing quality measured by the ratio of the standard deviation in fluid composition at a certain stage of mixing to the standard deviation at the mixer inlet; b) pressure drop factor measured by the ratio of pressure drop in a pipe without static mixing elements to the pressure drop in the same pipe but with static mixing elements, c) initial cost, and d) convenience of installation and easy maintenance.
A static mixing device, comprising a plurality of chambers, each chamber having an inlet and an outlet located in the opposite ends of a chamber displaced 180 degrees from each other, is described in U.S. Pat. No. 4,534,659 for xe2x80x9cPassive fluid mixing systemxe2x80x9d issued to Theodore A. Dourdeville and Anthony Lymneos. This simple design could provide low initial cost, low maintenance cost, and low pressure drop, but good mixing quality is difficult to achieve utilizing this device.
In U.S. Pat. No. 4,753,535 for xe2x80x9cMotionless mixerxe2x80x9d issued to Tony King, a static mixer is disclosed comprising axially overlapping mixing elements that induce counter-rotational angular velocities relative to the axial velocity of moving liquids. This design may contribute to undesirable increase in pressure drop.
In U.S. Pat. No. 5,137,369 for xe2x80x9cStatic mixing devicexe2x80x9d issued to John Hodan, a static mixing device is described comprising a stacked arrangement of plates, the latter having channels that split flow of liquid and guide it in the direction generally normal to the primary direction of flow. The mixer is modular in a sense that it comprises a plurality of those plates to achieve the desired mixing effect. The disadvantage of the design lays in high pressure drop and elevated maintenance cost because the above channels should be periodically cleaned to secure consistent mixing quality throughout the operating life of the apparatus.
In U.S. Pat. No. 5,843,385 for xe2x80x9cPlate-type chemical reactorxe2x80x9d issued to Jeffrey Dugan, a reactor is described, in which static mixing is achieved by a plurality of serially joined chambers containing flow-splitting means. The device is easy to maintain. However, thoroughness of mixing in some applications could turn out to be inadequate.
In U.S. Pat. No. 5,863,129 for xe2x80x9cSerial resin mixing devicesxe2x80x9d issued to Gary Smith, a disclosure is made to a family of inexpensive, easy to manufacture and easy to maintain static mixing devices, in which a multi-component liquid system flows through an elongated mixing chamber, the latter containing cylindrical mixing elements. This device is most suitable for such applications as mixing within spray guns or the like.
In U.S. Pat. No. 5,984,519 for xe2x80x9cFine particle producing devicesxe2x80x9d issued to Tadao Onodera et al., a device is disclosed where fluid flows through channels forming multiple high-speed streams, the streams colliding with each other creating pockets of turbulence. The device is claimed to be applicable only to conditions where high pressure could be applied to the fluid system.
In U.S. Pat. No. 6,000,418 for xe2x80x9cIntegrated dynamic fluid mixing apparatus and methodxe2x80x9d issued to Frederick Kern and William Syverson, a static mixer is described. The purpose of this mixer is to provide mixing means ensuring uniformity of cleaning and etching solutions used in semiconductor industry in the fabrication of integrated circuits on semiconductor wafers. The patented mixer employs multi-port venturi injectors. Such injectors provide easy to maintain and very accurate means of injecting required volumes of liquid chemicals into flow of carrier fluid. Using venturis generally entails higher power consumption if compared with mixers employing only static mixing elements.
A static mixing device comprising a plurality of axially extended helically twisted blades and impact bearing flat surfaces placed within a pipe is disclosed in U.S. Pat. No. 6,164,813 for xe2x80x9cStatic fluid mixing device with helically twisted elementsxe2x80x9d issued to Chiang-Ming Wang et al. The disadvantage of this device is in using the axial direction for static mixing, which does not allow proper usage of volume of the device, increasing pressure drop, as well as initial cost and maintenance cost.
It is to be understood that though known static mixers including those described above are very much suitable for their respective intended purposes, they do not achieve simultaneously consistent high quality mixing, low pressure drop, small initial capital investment, and easy maintainability, particularly in the applications related but not limited to thorough mixing of cleaning, etching, and polishing liquids used in semiconductor wafer manufacturing.
Therefore, an object of the present invention is to provide excellent quality of mixing by reducing the ratio of standard deviation of liquid system component characteristic such as concentration of a certain component in multi-component fluid system at the outlet of the static mixer to that at the inlet to the values close to zero. The exact value of the ratio depends upon a specific application.
Another object of the present invention is to provide as low pressure drop as possible for fluid components flowing through a mixer while maintaining high quality of mixing.
Yet another object of the present invention is to reduce the initial capital investment in manufacturing of the static mixer by means of simplifying and standardizing means used for dividing, cutting, and swirling flow of fluids being mixed.
Yet another object of the present invention is to substantially reduce maintenance costs by making modular means used for dividing, cutting, and swirling flow of fluids being mixed and having each mixing module easily cleanable.
One more object of the present invention is to make static mixer easy to assemble and disassemble.
Still one more object of the present invention is to provide means of static mixing suitable for fluid systems comprising Newtonian liquids, such as deionized high purity water, and Non-Newtonian fluids, more particularly pseudoplastic fluids such as slurries and some polymer solutions. Those skilled in the art appreciate that Newtonian fluids start to flow immediately after the pressure is applied and their strain is proportional to the stress, whereas pseudoplastic fluids start flowing only after stress exceeds a certain threshold value, and then their strain is proportional to the stress.
Yet another object of the present invention is to provide means of static mixing for Newtonian and Non-Newtonian fluids or their combination, particularly for purely viscous Non-Newtonian fluids as e.g. some polymer solutions like solution of carboxymethylcellulose in water. Purely viscous fluids start to flow immediately after pressure has been applied like Newtonian fluids, but unlike Newtonian fluids they have strain non-linearly dependent on stress.
Still another object of the present invention is to provide means of static mixing for suspensions whether Newtonian or Non-Newtonian or their combination.
Yet another object of the present invention is to provide means of uniformity control by means of static mixing for fluid systems employed in cleaning, etching, and polishing of surfaces of computer processing units and memory elements such as semiconductor wafers and compact disks, said means being inexpensive, easy to install, easily maintainable, and secure low pressure drop.
The above and other objects, features, and advantages of the invention are achieved by a static mixer that includes but not limited to a plurality of standardized mixing modules encased within a column where mixing takes place.
Those mixing modules create corresponding mixing zones within the column. In their entirety, the mixing zones provide excellent quality of mixing while keeping pressure drop low. In each mixing module static mixing elements are manufactured with a goal of achieving easy assembly and with another goal of avoiding stagnant fluid zones to avoid particulate sedimentation said avoidance of stagnant fluid zones increasing periods between cleanings.
The inlet mixing module has a chamber that consists of inlet means, outlet means, and an inlet mixing head, the inlet mixing head comprising a conical distributor placed in the center of the head, the distributor having a set of curved chutes and a set of curvilinear vanes extending from the bottom of said distributor in the direction of inner walls of the chamber. Fluid moving along the vanes acquires tangential component to its velocity. Another fluid is introduced into the chamber via tangential pairs of inlet conduits, each pair of the conduits being offset by 180xc2x0. The conduits are either manufactured together with the main body of the chamber by e.g. sheet metal forming, forging, or melting, or connected to the body of the chamber by welding or by any other suitable means. The conduits provide acceleration to the second fluid having their cross-sections decreased from inlet section inward. The direction of flow out of the conduits is close to normal relative to the direction of the flow of the first fluid created by the vanes and exiting into the chamber by means of a plurality of branch pipes. This arrangement causes vigorous mixing within the inlet mixing module dividing and cutting portions of both fluids and dissipating vortices by means of the swirling action.
Intermediate mixing module is generally placed downstream from the inlet mixing module. This module consists of at least one plate having a plurality of orifices and/or channels drilled or otherwise machined, the orifices and/or channels splitting flow of fluid premixed by the inlet mixing module. The orifices and/or channels form jets that promote mixing in the flow of mixture directed downward from the plate. The intermediate mixing module could comprise stacks of plates, each plate having orifices and/or channels drilled or otherwise machined in such a way as to offset them from plate to plate causing jet impingement upon the lower plate, which further enhances mixing.
The outlet mixing module is placed at the bottom end of the column. This mixing module comprises inlet means, outlet means, and a chamber, the chamber comprising an outlet mixing head that is very much similar to the mixing head employed in the inlet mixing module. It also comprises a conical distributor in its center and a set of curvilinear vanes extending from the bottom of that distributor in the direction of inner walls of the chamber. The outlet mixing module lacks conduits present in the inlet mixing module. The conduits have been used in the inlet mixing module for initial mixing of two fluids entering the column through respective inlet ports. Because the outlet mixing module is employed for further mixing of the already premixed fluid system, there is no need anymore for such conduits. The curvilinear vanes employed in the outlet mixing module are similar to, but not exactly the same as the vanes employed in the inlet mixing module. The vanes of the outlet mixing module might have different angle distribution along the vane if compared with the vanes of the inlet mixing module because they enhance mixing of the already premixed fluid system, while the vanes of the inlet mixing module should be able to sustain vigorous mixing at the initial stage of fluids entering the column. Also, the number of vanes could be different in inlet and outlet mixing heads. However, the similarity of design of the heads belonging to the inlet and to the outlet mixing modules is certainly an advantage because it facilitates assembling, disassembling, and cleaning operations.
The present invention is not limited to the usage of three mixing modules. The number of mixing modules could be less or more depending on the circumstances of the mixing process. The intermediate mixing module comprising at least one plate with orifices and/or channels could be installed repeatedly along the column to achieve desired mixing of components of the fluid system. The outlet mixing module also could be installed not only at the outlet portion of the column as described above, but also in various cross-sections of the middle part of the column to further enhance mixing. Moreover, columns could be interconnected providing even more thorough mixing.